1. Field of the Invention
This invention is related to the field of clocking of integrated circuits and, more particularly, to multiplying the frequency of a clock.
2. Description of the Related Art
Integrated circuits may include clock multiplier circuits. Generally, the clock multiplier circuit in an integrated circuit is used for multiplying the frequency of a clock input (or inputs) to the integrated circuit to generate one or more clocks for internal use within the integrated circuit. The clock multiplier may be used to allow lower frequency clocks to be supplied to the integrated circuit, while still allowing the higher frequency operation within the integrated circuit.
One typical method for building a clock multiplier circuit is using a phase-locked loop (PLL). Generally, a PLL requires a certain amount of time (lock time) to adapt if the input clock frequency is changed during operation or if the multiplier ratio (between the output of the clock multiplier circuit and the input clock signal) is changed during operation. Similarly, if the clock input is stopped, the PLL may require a certain amount of time to stop. If the input clock is restarted, the PLL may require the lock time to restart in a predictable fashion. While a maximum lock time may be specified, the actual lock time may not be predetermined. Variations in lock time may occur due to process parameters in the fabrication process, operating temperature, supply voltage, etc.
In some cases, the lack of determinism in the operation of the PLL may be problematic. For example, during testing of the integrated circuit, determinism in the clocking may be desired in order to generate test results that may be matched with expected results. If the number of clock periods that have elapsed in the integrated circuit is not deterministic, it may be difficult to observe state in the integrated circuit and verify that the state is correct as expected by the test.